The use of solid-state materials as heaters is well known in the industry. U.S. Pat. No. 4,236,065 to Yashin et al. is one of many that describe this technology. In particular, there are in use today two types of solid-state heaters, positive temperature coefficient of resistance (PTCR) and negative coefficient of resistance (NTCR) heaters.
It is well known in the industry that a heater can be made with either of the two materials formed as a solid pellet with flat surfaces on opposite sides. The flat surfaces are coated with a metal that forms an electrical bond with the PTCR or NTCR body. When opposite polarity electrical connections are made to the two opposed metallic surfaces, current will flow through the body of the material and the resistive characteristics of the substance produces heat.
In the industry, electrical connection may be made by three methods. The first and less common method is to use a high temperature soldered connection or a brazed connection directly to the metal surfaces of the either the NTCR or the PTCR pellet. The second way is to attach two electrical lead wires each to its own metal plate, with each metal plate having approximately the same or even larger surface area than the heater, then to attach the metal plates, one to each side of the PTCR or NTCR body. The third is to attach each lead wire to metal terminals, and by some method apply sufficient force so the terminals press against the two metal surfaces, one terminal on one side and the second on the opposite side. Of course, the application of the proper sized electrical power source will produce the desired heating effect in each of the three cases above.
One method of encapsulating any one of the heater subassemblies described above is to use a molding material that produces either a soft body or a rigid body solid-state heater assembly. For example the use of silicone rubber results in a soft heater body and some epoxy materials can produce a more rigid body. Both methods of encapsulating various types of electronic components are well known. See “Electric Heating Elements”, 1995 edition, P.111–P.125, Fritz Eichenauer GmbH+Co.KG.
One type of PTCR heater is made to conduct heat to the inner surfaces of a metal well that is shaped as a cylinder with one end open and the other end closed. The outer surfaces of the well are in contact with a media to be heated. In order for the PTCR material to produce heat, electrical power must be delivered by leads that contact the metal surfaces as outlined above. In the present day technology, spring type metal terminals are attached to each of two lead wires using conventional crimping techniques. The terminals, with wires attached, are inserted into a heater case made of a ceramic material that contains a previously inserted solid pellet of PTCR material with metallic surfaces. The ceramic has a special shaped hole which centers the PTCR pellet and presses the spring type terminals against the pellet's metal surfaces. A “potting” compound is inserted into the open end of the ceramic shell filling the cavity and sealing the heater and the insulated lead wires. The hard shell of the ceramic heater must be sufficiently smaller than the inside dimensions of the metal well that it is to be placed in to prevent overlapping production tolerances from creating an interference.
To ensure the desired heating effect, a high temperature grease is used to coat the ceramic heater body and contact the well's inner surface, thus permitting heat transfer from the rigid ceramic case to the well. Without this grease, the PTCR heater would not produce the necessary temperature rise in the media being heated and could possibly result in system failure.
The electrical connection methods exhibited by the prior art are difficult and expensive. Brazing involves fluxes and high temperature solders having heavy metals which are dangerous and their use in manufacturing is banned in some areas.
For the ceramic case type PTCR heater noted above, there are a number of other issues that create problems. Though the rigid ceramic case provides a means to hold spring type terminals securely to the metallic surfaces of a heater pellet, the nature of ceramic bodies is that it is virtually impossible to manufacture heaters without some being too large to be inserted in a well or the like and some so small they would not contact the inner well surfaces sufficiently to properly conduct the heat generated. As mentioned previously, to ensure all heaters made with ceramics will fit into the intended openings all must be made so that the largest tolerance heaters will fit into the smallest tolerance openings.
As noted in the prior art for ceramic case heaters, the practice of manufacturing undersize ceramic cases and using a grease to promote heat conduction is costly as well as undesirable because the grease tends to cause a dirty work environment. Over time and with the heaters under operating conditions, it is also possible for the grease to flow out of the shell being heated thus reducing the amount of heat conducted. Further, any remaining thermal grease will harden and crack over time at elevated temperatures. This phenomenon has the deleterious effect of reducing the temperature of the media being heated.
In light of these problems noted above, a need has developed for a solid-state heater that is economical to produce and has a soft or semi-rigid body that will expand when it generates heat so as to tightly contact the inner surface of the well to be heated. Such a heater will eliminate the need for expensive and messy thermally conductive grease used today. The new invention described below addresses the above need and can also be used in the construction of ceramic case solid-case heaters.